RE: [gnomemm] custom canvas item

[Murray Cumming Comneon com]

> From: Marcus Brinkmann [mailto:Marcus Brinkmann ruhr-uni-bochum de] 
> It would be nice to have a 
> Gnome::Canvas::Item (Gnome::Canvas::Group &parentx) 
> constructor that does the above wrapping of the C object in 
> an opaque way.

If that would help then please do submit a patch. It could be applied in an
unstable API branch for the future.

> Now, I thought that to make my item behave like the other 
> items, it would be nice to use the property system.  However, 
> adding the property proxies doesn't appear to be enough, 
> because the underlying gobject doesn't know anything about 
> the properties:
> 
> class AudioFileItem
> {
>   ...
> 
> public:
>   Glib::PropertyProxy<Glib::ustring> property_fill_color ();
> };
> 
> Glib::PropertyProxy<Glib::ustring> 
> AudioFileItem::property_fill_color()
> {
>   return Glib::PropertyProxy<Glib::ustring> (this, "fill_color"); }
> 
> This results in a run-time warning:
> 
> (main:2738): GLib-GObject-WARNING **: gobject.c:1032: object 
> class  tkmm__GnomeCanvasItem' has no property named ill_color'

Glib::PropertyProxy is for wrapping pre-existing GTK+ properties. You
probably need to use the C API to add properties.

> It would be nice to have a somewhat automatic way to register 
> a new object class and register its properties.

OK. Please add a bug glibmm 2.4 bug about that. Any suggestions for API
would be useful, after you have looked at the C API.

>  However, I 
> wouldn't mind a manual way to do that, too.  I am quite a 
> Glib/Gdk/Gtk/Gnome newbie, so if you have an example or a 
> suggestion how to do that, I would appreciate it.

You should look at some of the _init functions in GTK+ or libgnomecanvas, I
suppose.

> I also found a bug, but I am not sure if I didn't something 
> wrong. If in the above class I would change the order and 
> first list the canvas item, then the audiofile, and the 
> audiofile constructor throws an exception, then the 
> application crashes in the Item destructor:
> 
> (main:2772): GnomeCanvas-CRITICAL **: file gnome-canvas.c: line 3721
> (gnome_canvas_request_redraw): assertion  NOME_IS_CANVAS 
> (canvas)' failed
>    
> #0  0x406a0add in gnome_canvas_item_construct () from 
> /usr/lib/libgnomecanvas-2.so.0 #1  0x40af0429 in 
> g_object_class_list_properties () from 
> /usr/lib/libgobject-2.0.so.0 #2  0x4038eb43 in 
> Gtk::Object::_destroy_c_instance() () from 
> /usr/lib/libgtkmm-2.0.so.1 #3  0x4038ee0a in 
> Gtk::Object::destroy_() () from /usr/lib/libgtkmm-2.0.so.1 #4 
>  0x400b17d1 in Gnome::Canvas::Item::~Item() () from 
> /usr/lib/libgnomecanvasmm-2.0.so.1
> 
> I don't have debugging symbols installed, but I can probably 
> make a small test case if useful.

Yes, please. We like small simple-as-possible test cases.

Murray Cumming
murrayc usa net
www.murrayc.com